Hydrogenation of unsaturated carbon structures using hydrogen gas and a metal catalyst is a reaction well known to the chemical industry. However, the use of molecular hydrogen poses a serious risk of fire or explosion, with subsequent formation of toxic by-products.
Reduction reactions which employ an organic molecule that functions as the hydrogen donor in the presence of a catalyst are also known in the art and commonly referred to as catalytic transfer hydrogenation methods. The catalytic transfer hydrogenation reaction may be generalized as follows: ##STR1##
In principle, the donor compound can be any organic compound whose oxidation potential is sufficiently low that hydrogen transfer can occur under relatively mild conditions. For example, at temperatures greater than about 300.degree. C., benzene serves as a hydrogen acceptor and can be reduced to cyclohexane.
Conventional catalytic transfer hydrogenation methods have shown only little commercial potential, generally owing to poor yields and long reaction times, and as a result of the very successful exploitation of the aforementioned methods employing molecular hydrogen with metallic catalysts and hydrides. Cortese et al, J. Org. Chem., 43, 3985 (1978), have disclosed the reductive elimination of halogens from a number of halogenated compounds employing triethylammonium halogens, a temperature of 100.degree. C., triethylammonium formate as a hydrogen donor, a reaction time of 6 hours and a palladium catalyst. While some dehalogenation was achieved, the reaction was incomplete.
Bamfield et al, Synthesis, 537 (1978), have disclosed the use of an aqueous alkaline sodium formate solution, a palladium catalyst, a surfactant, a 32 percent hydroxide solution and a temperature of 95.degree. C. to remove halogens from compounds in the synthesis of symmetrical biphenyl. The disclosed reaction resulted in only moderate yields of biphenyl. Wiener et al, J. Org. Chem., 50, 21 (1991) have described methods for catalytic transfer hydrogenation of aryl halides for producing the corresponding arenes. A palladium catalyst effected the transfer hydrogenolysis of the aryl halides using potassium and sodium formate as the hydrogen donor at a temperature of 60.degree. C. and in the presence of an initial amount of water.
In other studies, Crawford et al, Trans. Faraday Soc., 58, 2452 (1962), demonstrated the reduction of alkynes to cis-alkenes by employing molecular hydrogen and palladium catalyst, and Wiener et al have reported on the application of aqueous formate salts as hydrogen donors, Int. J. Hydrogen Energy, 14, pp 365-370 (1989).
Most of the elements that have proved to be valuable catalysts for catalytic transfer reductions ate a part of the second transition series in the periodic table. Salts and complexes of Pd, Pt, Ru, Ir, Rh, Fe, Ni, and Co, and particularly of palladium, have all been used primarily as heterogenous catalysts. The most active catalysts reported for heterogeneous transfer reduction are based on palladium metal. However, the transition series metal catalysts employed in typical catalytic transfer hydrogenation reactions are costly, and new technology must be developed and implemented for their recovery. Strenuous efforts have been undertaken to find catalysts from less expensive metals for use in catalytic hydrogen transfer reduction reactions.
Thus, if catalytic transfer hydrogenation methods are to be effective for the synthesis of chemical compounds, they must be able to employ low cost hydrogen donors and low cost, non-toxic catalysts which do not require extensive recovery treatment. Additionally, the methods must reduce synthesis time as compared with existing catalytic transfer hydrogenation processes, and they must produce high yields of the desired products. It would also be advantageous for such methods to be effected in pressurized, closed and non-pressurized reaction systems. Thus, a need exists for improved catalytic transfer hydrogenation processes for producing chemical compounds.